Electric eel
The electric eels are a genus, Electrophorus, of neotropical freshwater fish from South America in the family Gymnotidae, of which they are the only members of the subfamily Electrophorinae. They are known for their ability to stun their prey by generating electricity, delivering shocks at up to 860 volts. Their electrical capabilities were first studied in 1775, contributing to the invention of the electric battery in 1800.
Despite their name, electric eels are not closely related to the true eels but are members of the electroreceptive knifefish order Gymnotiformes. This order is more closely related to catfish. In 2019, electric eels were split into three species: for more than two centuries before that, the genus was believed to be monotypic, containing only Electrophorus electricus.
They are nocturnal, obligate air-breathing animals, with poor vision complemented by electrolocation; they mainly eat fish. Electric eels grow for as long as they live, adding more vertebrae to their spinal column. Males are larger than females. Some captive specimens have lived for over 20 years.
Evolution
Taxonomy
When electric eels were described by Carl Linnaeus in 1766, based on early field research by Europeans in South America and specimens sent back to Europe for study, he used the name Gymnotus electricus, placing it in the same genus as Gymnotus carapo. He noted that the fish is from the rivers of Surinam, that it causes painful shocks, and that it had small pits around the head.In 1864, Theodore Gill moved the electric eel to its own genus, Electrophorus.
The name is from the Greek ήλεκτρον, and φέρω, giving the meaning 'electricity bearer'. In 1872, Gill decided that the electric eel was sufficiently distinct to have its own family, Electrophoridae. In 1998, Albert and Campos-da-Paz lumped the Electrophorus genus with the family Gymnotidae, alongside Gymnotus, as did Ferraris and colleagues in 2017.
In 2019, C. David de Santana and colleagues divided E. electricus into three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electrical ability. The three species are E. electricus, and the two new species E. voltai and E. varii. However, this revision did not address Electrophorus multivalvulus, which was described from the Peruvian Amazon by Nakashima in 1941. Therefore, E. varii may be a junior synonym of E. multivalvulus and has been regarded as such by some biologists.
Phylogeny
Electric eels form a clade of strongly electric fishes within the order Gymnotiformes, the South American knifefishes. Electric eels are thus not closely related to the true eels. The lineage of the Electrophorus genus is estimated to have split from its sister taxon Gymnotus sometime in the Cretaceous. Most knifefishes are weakly electric, capable of active electrolocation but not of delivering shocks. Their relationships, as shown in the cladogram, were analysed by sequencing their mitochondrial DNA in 2019. Actively electrolocating fish are marked with a small yellow lightning flash. Fish able to deliver electric shocks are marked with a red lightning flash.Species
There are three described species in the genus, not differing significantly in body shape or coloration:- Electrophorus electricus This, the type species, has a U-shaped head, with a flattened skull and cleithrum.
- Electrophorus voltai This species is the strongest bioelectricity generator in nature, capable of generating 860 V. Like E. electricus, this species has a flattened skull and cleithrum but the head is more egg-shaped.
- Electrophorus varii Compared to the other two species, this one has a thicker skull and cleithrum but the head shape is more variable.
E. varii appears to have diverged from the other species around 7.1 mya during the late Miocene, while E. electricus and E. voltai may have split around 3.6 mya during the Pliocene.
Ecology
The three species have largely non-overlapping distributions in the northern part of South America. E. electricus is northern, confined to the Guiana Shield, while E. voltai is southern, ranging from the Brazilian shield northwards; both species live in upland waters. E. varii is central, largely in the lowlands. The lowland region of E. varii is a variable environment, with habitats ranging from streams through grassland and ravines to ponds, and large changes in water level between the wet and dry seasons. All live on muddy river bottoms and sometimes swamps, favouring areas in deep shade. They can tolerate water low in oxygen as they swim to the surface to breathe air.Electric eels are mostly nocturnal. E. voltai mainly eats fish, in particular the armoured catfish Megalechis thoracata. A specimen of E. voltai had a caecilian, Typhlonectes compressicauda, in its stomach; it is possible that this means that the species is resistant to the caecilian's toxic skin secretions. E. voltai sometimes hunts in packs; and have been observed targeting a shoal of tetras, then herding them and launching joint strikes on the closely packed fish. The other species, E. varii, is also a fish predator; it preys especially on Callichthyidae and Cichlidae.
File:Carte du nord de l'Amérique du sud avec répartition de spécimens de 3 espèces d'anguilles électriques electrophorus.png|thumb|upright=2|center|Map of the northern part of South America showing distribution of specimens of the three species of Electrophorus: E. electricus ; E. voltai ; E. varii.|alt=Map of South America showing distribution of the three species of electric eel
Biology
General biology
Electric eels have long, stout bodies, being somewhat cylindrical at the front but more flattened towards the tail end. E. electricus can reach in length, and in weight. The mouth is at the front of the snout, and opens upwards. They have smooth, thick, brown-to-black skin with a yellow or red underbelly and no scales. The pectoral fins each possess eight tiny radial bones at the tip.They have over 100 precaudal vertebrae, whereas other gymnotids have up to 51; in total, there can be as many as 300 vertebrae.
There is no clear boundary between the tail fin and the anal fin, which extends much of the length of the body on the underside and has over 400 bony rays. Electric eels rely on the wave-like movements of their elongated anal fin to propel themselves through the water.
Electric eels get most of their oxygen by breathing air using buccal pumping. This enables them to live in habitats with widely varying oxygen levels, including streams, swamps, and pools. Uniquely among the gymnotids, the buccal cavity is lined with a frilled mucosa which has a rich blood supply, enabling gas exchange between the air and the blood. About every two minutes, the fish takes in air through the mouth, holds it in the buccal cavity, and expels it through the opercular openings at the sides of the head. Unlike in other air-breathing fish, the tiny gills of electric eels do not ventilate when taking in air. The majority of carbon dioxide produced is expelled through the skin. These fish can survive on land for some hours if their skin is wet enough.
Electric eels have small eyes and poor vision. They are capable of hearing via a Weberian apparatus, which consists of tiny bones connecting the inner ear to the swim bladder. All of the vital organs are packed near the front of the animal, taking up only 20% of space and sequestered from the electric organs.
Electrophysiology
Electric eels can locate their prey using electroreceptors derived from the lateral line organ in the head. The lateral line itself is mechanosensory, enabling them to sense water movements created by animals nearby. The lateral line canals are beneath the skin, but their position is visible as lines of pits on the head. Electric eels use their high frequency-sensitive tuberous receptors, distributed in patches over the body, for hunting other knifefish.File:Biotechnology, systems biology, artificial cells.jpg|thumb|center|upright=1.5|Electric eel anatomy: first detail shows stacks of electrocytes forming electric organs. Second detail shows an individual cell with ion channels and pumps through the cell membrane; A nerve cell's terminal buttons are releasing neurotransmitters to trigger electrical activity. Final detail shows coiled protein chains of an ion channel.
Electric eels have three pairs of electric organs, arranged longitudinally: the main organ, Hunter's organ, and Sachs' organ. These organs enable electric eels to generate two types of electric organ discharge: low- and high-voltage. The organs are made of electrocytes, modified from muscle cells. Like muscle cells, the electric eel's electrocytes contain the proteins actin and desmin, but where muscle cell proteins form a dense structure of parallel fibrils, in electrocytes they form a loose network. Five different forms of desmin occur in electrocytes, compared to two or three in muscle, but its function in electrocytes remained unknown as of 2017.
Potassium channel proteins involved in electric organ discharge, including KCNA1, KCNH6, and KCNJ12, are distributed differently among the three electric organs: most such proteins are most abundant in the main organ and least abundant in Sachs's organ, but KCNH6 is most abundant in Sachs's organ. The main organ and Hunter's organ are rich in the protein calmodulin, involved in controlling calcium ion levels. Calmodulin and calcium help to regulate the voltage-gated sodium channels that create the electrical discharge. These organs are also rich in sodium potassium ATPase, an ion pump used to create a potential difference across cell membranes.
The maximum discharge from the main organ is at least 600 volts, making electric eels the most powerful of all electric fishes. Freshwater fishes like the electric eel require a high voltage to give a strong shock because freshwater has high resistance; powerful marine electric fishes like the torpedo ray give a shock at much lower voltage but a far higher current. The electric eel produces its strong discharge extremely rapidly, at a rate of as much as 500 Hertz, meaning that each shock lasts only about two milliseconds. To generate a high voltage, an electric eel stacks some 6,000 electrocytes in series in its main organ; the organ contains some 35 such stacks in parallel, on each side of the body. The ability to produce high-voltage, high-frequency pulses in addition enables the electric eel to electrolocate rapidly moving prey. The maximum electric current delivered during each pulse can reach about 1 ampere at 500V.
File:Impedance matching in electric fishes.svg|thumb|center|upright=2.6|alt=diagram showing why freshwater electric fish need to produce a high voltage|Impedance matching in strongly electric fishes. Since freshwater is a poor conductor, limiting the electric current, electric eels need to operate at high voltage to deliver a stunning shock. They achieve this by stacking a large number of electrocytes, each producing a small voltage, in series.
It remains unclear why electric eels have three electric organs but produce only two types of discharge: to electrolocate or to stun. In 2021, Jun Xu and colleagues stated that Hunter's organ produces a third type of discharge at a middle voltage of 38.5 to 56.5 volts. Their measurements indicate that this is produced just once, for less than 2 milliseconds, after the low-voltage discharge of Sachs's organ and before the high-voltage discharge of the main organ. They believed that this is insufficient to stimulate a response from the prey, so they suggested it may have the function of coordination within the electric eel's body, perhaps by balancing the electrical charge, but state that more research is needed.
When an electric eel identifies prey, its brain sends a nerve signal to the electric organ; the nerve cells involved release the neurotransmitter chemical acetylcholine to trigger an electric organ discharge. This opens ion channels, allowing sodium to flow into the electrocytes, reversing the polarity momentarily. The discharge is terminated by an outflow of potassium ions through a separate set of ion channels. By causing a sudden difference in electric potential, it generates an electric current in a manner similar to a battery, in which cells are stacked to produce a desired total voltage output. It has been suggested that Sachs' organ is used for electrolocation; its discharge is of nearly 10 volts at a frequency of around 25 Hz. The main organ, supported by Hunter's organ in some way, is used to stun prey or to deter predators; it can emit signals at rates of several hundred hertz. Electric eels can concentrate the discharge to stun prey more effectively by curling up and making contact with the prey at two points along the body. It has also been suggested that electric eels can control their prey's nervous systems and muscles via electrical pulses, keeping prey from escaping, or forcing it to move so they can locate it, but this has been disputed. In self-defence, electric eels have been observed to leap from the water to deliver electric shocks to animals that might pose a threat. The shocks from leaping electric eels are powerful enough to drive away animals as large as horses.